Electronic component heat sink attachment using a canted coil spring

Abstract: A thermal attachment assembly for heat generating electronic devices which uses a canted coil spring to apply pressure directly against electronic components. The attachment assembly includes a heat sink housing in which the electronic components are pressed against an electrically insulating, thermally conductive film positioned against a surface of a shoulder or side wall of the heat sink housing. The force of the canted coil spring can be counterbalanced by a cover for the housing, or by an abutment that is either fixed in the housing or floats between attachment and an assembly at each of opposite sides of the housing.

Description: This invention relates to the attachment of printed circuit board-mounted, heat generating electrical components to heat sinks. In particular, to the attachment of such electrical components which have heat transfer tabs to heat sink housings using spring type members. One method by which tabbed printed circuit board-mounted, heat generating electrical components have been attached to heat sinks has involved individually attaching each device to the heat sink with a screw through the hole in the tab of the electronic device, and utilizing an electrically insulating thermally conductive film between the heat sink and the device tab. A nut is tightened onto the screw to provide the compressive force required to provide adequate compression of the film to maximize heat transfer from the device to the heat sink through the film. There are many variations of this technique, such as using a threaded hole in the heat sink rather than a nut. Problems arise with this general manner of attachment because the tension created by the screw may be relieved by material creep in the insulating washer or grommet, or the screw may back out under certain operating conditions, such as where the device is subject to vibration or temperature cycling. Another disadvantage is that the large number of parts that must be manually assembled when there are a number of heat generating components in an assembly, and it is difficult to automate the production of such attachments for high volume manufacturing. In addition to the various uses of attachment screws, numerous attachment techniques have been developed which use spring members. A number of these attachments use clips with multiple spring fingers, as can be seen by reference to U.S. patents 4,922,601; 4,891,735; 4,872,089; 4,845,590; 4,707,726; 4,674,005; 4,605,986; and 4,479,140, for example. Difficulties are encountered with these techniques for various reasons. To achieve a compact assembly, the springs must achieve high forces at low deflections, and this means that the springs must be manufactured specifically for an individual device and changing the device may cause the spring to exert too much or too little force since stiff springs have force constants which create a very narrow range of deflections within which the proper pressure will be applied. Furthermore, there is often a need for special tools to insure that the device or the spring is not damaged during assembly. Historically, electrically insulating and thermally conductive pads require 2.068 - 3.447 MPa (300-500 psi) for effective heat transfer, but, this requires a material to be disposed between the spring and the device to distribute the spring force on the device to prevent the spring from damaging the device. Some arrangements require that the heat sink be a separate part from the housing, and others require numerous unique parts that need to be manually assembled. Canted oil springs have the attribute that, unlike conventional springs which apply a force which varies as a function of deflection, they apply an essentially constant force over a large deflection range. This type of spring is well known for use in electronic devices. Most commonly, they are utilized as a means to hold a printed circuit board in place in proper alignment. In this regard, reference can be made to U.S. patents 5,092,781 and 5,069,627, as well as to page 3.1B-2 of Bal Seal Engineering Company catalog no. LE-44D. Additionally, canted springs are known for use as electrical and thermal conductors. For example, in U.S. patent 4,993,482, a thermally conductive, canted coil spring is used to provide a low force, thermal path between heat generating electronic components and a heat sink in close proximity. In this arrangement, the spring is flattened to increase the contact between the canted spring and the components and heat sink and thereby decrease the thermal resistance. Thus, there is a need for a heat sink attachment which will enable the use of flow force springs in order to increase the range of permissible spring deflections, thereby increasing the tolerances and the variety of heat sinks which may be attached, and to eliminate the need for intervening materials for distributing the spring force. Additionally, a need also exists for an arrangement which will not require numerous parts that require manual assembly. In keeping with the foregoing, primary objects of the present invention involve the provision of an attachment between numerous types of power generating electrical devices and a housing which also serves as a heat sink while minimizing the number of parts required, facilitating manual or automated assembly, achieving a minimal packaged volume, and accepting of parts with a greater degree of dimensional tolerance. These and other objects of the present invention are achieved by an electronic component heat sink arrangement as defined in claim 1. In accordance with preferred embodiments, the compressive force can be applied to the spring via a cover for a housing serving as the heat sink or by an abutment that is mounted in the housing. One or more canted springs are attached to the cover or the abutment and act on one or more electrical components that are carried on a circuit board and have had their leads bent to minimize stresses. These and further objects, features and advantages of the present invention will become apparent from the following description when taken in connection with the accompanying drawings which, for purposes of illustration only, show several embodiments in accordance with the present invention. In the drawings: Fig. 1 is a cross section through an assembly utilizing the attachment device of the present invention, Fig. 2 is an exploded view of the assembly shown in Fig. 1, Fig. 3 is a cross-sectional view of a modified assembly utilizing an attachment device in accordance with the present invention, Fig. 4 is a top view of a portion of the Fig. 3 assembly in the area in which the canted coil springs are supported, and Fig. 5 is a view like that of Fig. 4, but of a modified embodiment. Figs. 1 and 2 show a preferred embodiment of an attachment in accordance with the present invention in which a housing 1 serves as a heat sink for at least one power generating electronic component 3, such as T0220's and leaded D-Paks, that have a body portion 3a from which a heat transfer tab 3b and leads 3c extend. To conduct heat away from the heat generating components, the housing is formed of metal of adequate thermal conductivity, such as aluminium or a copper alloy. Canted coil springs 5 are attached to the underside of a cover 7 for the housing 1 and act to hold the components 3 flush against an electrically insulative, thermally conductive film 9 that has been applied on the top wall surface of a supporting shoulder 1a, when the cover is attached to the housing. The electrically insulative, thermally conductive film 9 is formed of, for example, polyimide film beneath a layer of boron-nitride filled silicon which will efficiently transfer heat away from mounted components without requiring a high clamping pressure. Furthermore, the film 9 is, preferably, adhesive-backed, so that it can be easily mounted to the wall surface of shoulder 1a of housing 1. The body portion 3a or transfer tab 3b of the electronic component is held against the film 9 by the canted coil springs 5 held against the electronic components 3 with a spring deflection that is within a substantially constant spring force range (e.g., 137.9 kPa - 344.7 kPa (20-50 psi)) of the canted spring 5 when the cover 7 is attached to the housing 1, despite variations in the assembly tolerances or use of different components. The leads 3c, by which the components 3 are electrically connected to the circuit board 11, are bent or formed to minimize the force required by the springs 5 to move the heat transfer tab 3b or back of the body 3a against the film 9 and to minimize the stresses in the leads after this displacement. In this embodiment, the circuit board 11 is oriented parallel to the bottom wall of housing 1 and top surfaces of its shoulder 1a, so that the electronic components connected to a top side of the circuit board 11 with the leads 3c being bent at a right angle to the circuit board 11, so that the electronic components 3 are pressed on top of the shoulder 1a laterally outwardly of a peripheral edge of the printed wiring board within a notch formed in board 11 that is matched to the shoulder 1a at opposite sides of the circuit board 11. Alternatively, as shown in the embodiments of Figs. 3-5, where the row of electronic components 3 are pressed on a side wall 1b of the housing 1, instead of cover 7, an abutment member 15 is utilized to provide the necessary counterbalancing effect. Abutment member 15, preferably, is a plastic supporter fixed to the bottom wall of housing 1. As shown in Fig. 4, a single canted coil spring 5 can be provided for each electronic component 3, each canted coil spring 5 being supported by the abutment member 15 oriented parallel to the respective electronic component 3, i. e., vertically. Alternatively, as with the embodiment of Figs. 1 and 2, as shown in Fig. 5, a single canted coil spring 5 can be provided for a plurality of electronic components 3, in which case the abutment member 15 supports the canted coil spring 5 oriented parallel to the row of electronic components 3, i. e., horizontally. If electronic components 3 are provided, in the manner shown in Fig. 3, at opposite sides of the housing 1, as is the case shown in Figs. 1 and 2, a single abutment member 15, substantially spanning the distance between the attachment areas, or a pair of abutment members 15, one for each side, can be fixed to the bottom wall of the housing 1 for supporting and opposing the force of the canted coil springs 5 at each side wall 1b. As shown in Fig. 3, leads 3c, by which the components 3 are electrically connected to the circuit board 11, are bent or formed into a generally S-shaped configuration to minimize the force required by the springs 5 to move the heat transfer tab 3b or back of the body 3a against the film 9 and to minimize the stresses in the leads 3c after this displacement. Instead of the cover 7 having an inwardly projecting portion 7a which carries the canted coil springs 5, the coil springs 5 could be mounted to spring supports attached to the underside of a flat cover. As such, we do not wish to be limited to the details shown and described herein and intend to cover all such changes and modifications as are encompassed by the scope of the appended claims. The present invention will find a wide range of application to devices in which various electronic components, which generate amounts of heat which cannot be adequately dissipated without using a heat sink, are enclosed in a housing.

Claims: Electronic component heat sink arrangement comprising a housing (1) formed of a heat sinking material and containing a printed circuit wiring board (11) and a plurality of heat generating electronic components (3), at least one canted coil spring (5) for pressing the electronic components (3) into a nonelectrically conductive, heat exchange relationship with an inner wall surface of the housing (1), and means (17; 15) for holding the canted coil spring (5) against the electronic components (3) with a spring deflection that is within a substantially constant spring force range of the canted spring (5). Electronic component heat sink arrangement according to claim 1, wherein said canted coil spring (5) is constructed to apply a pressure in a range of 137.9 - 344.7 kPa (20-50 psi) to a full range of differing sizes of electronic components (3). Electronic component heat sink arrangement according to claim 1 or 2, wherein a supporting shoulder (1a) is provided on a bottom wall of the housing (1) and the canted coil spring (5) presses the electronic components (5) against a top surface of said supporting shoulder (1a). Electronic component heat sink arrangement according to any preceding claim, wherein the means for holding the canted coil spring (5) against the electronic components (3) is formed by a cover (7) for said housing (1), said canted coil spring (5) being compressed between the electronic components (3) and a portion of the cover (7) which faces said inner wall surface of the housing (1). Electronic component heat sink arrangement according to any preceding claim, wherein the housing (1) is provided with a pair of supporting shoulders (1a) on the bottom wall thereof, a respective canted coil spring (5) pressing a row of said electronic components (3) against a top surface of each said supporting shoulder (1a). Electronic component heat sink arrangement according to any preceding claim wherein the or each canted coil spring (5) is carried by the cover (7), and, optionally, wherein the cover (7) has an inwardly projecting portion upon which the canted coil springs (5) are carried. Electronic component heat sink arrangement according to any preceding claim, wherein an electrically insulative, thermally conductive film (9) is disposed on the inner wall surface of the housing (1) or on the top wall surface of the supporting shoulder (1a) respectively, the electronic components (3) being in said nonelectrically conductive, heat exchange relationship with the inner wall surface of the housing (1) or said top wall surface via said film (9). Electronic component heat sink arrangement according to any preceding claim, wherein said printed wiring board (11) is oriented parallel to said inner wall surface, wherein said electronic components (3) are connected to a top side of said printed wiring board (11) via electrically conductive leads (3c), said leads (3c) being bent at a right angle to said printed wiring board (11) and the electronic components (3) being pressed against said inner wall surface laterally outwardly of a peripheral edge of the printed wiring board (11). Electronic component heat sink arrangement according to any preceding claim, wherein said inner wall surface is a side wall (1b) of the housing (1) and, preferably, wherein the means for holding the at least one canted coil spring (5) against the electronic components (3) is formed by an abutment member (15) in said housing (1), said canted coil spring (5) being compressed between the electronic components (3) and said abutment member (15). Electronic component heat sink arrangement according to claim 9, wherein a canted coil spring (5) is provided for each electronic component (3), the abutment member supporting (15) each canted coil spring (5) oriented parallel to the respective electronic component (3), and/or wherein the abutment member (15) is fixed to the housing (1), and/or wherein a single canted coil spring (5) is provied for a plurality of electronic components (3), the abutment member (15) supporting the canted coil spring (5) oriented parallel to a row of said electronic components (3), and/or wherein said inner wall surface comprises an inner side wall surface (1b) at opposite sides of the housing (1), and wherein canted coil springs (5) are provided for pressing electrical components (3) against each of the inner side wall surfaces (1b). Electronic component heat sink arrangement according to claim 9 or 10, wherein the electrical components (3) have conductive leads (3c) by which they are connected to a circuit board (11), said leads (3c) having a generally S-shaped configuration for minimizing the force required by the springs (5) to move the electrical components (3) against said inner wall surfaces and for minimizing the stresses in the leads (3c) after the electrical components (3) have been moved against said inner wall surfaces.

Inventors: CASPERSON PAUL G, CASPERSON, PAUL G.
Applicants: CUMMINS ENGINE CO INC, CUMMINS ENGINE COMPANY, INC.
Application year: 1994
Main classification: H05K7
IPC codes: H01L23, H05K7, H05K5